16 Introduction to Descriptors

TopLink uses descriptors to store the information that describes how an instance of a particular class can be represented by a data source. Descriptors own mappings that associate class instance variables with a data source and transformation routines that are used to store and retrieve values. As such, the descriptor acts as the connection between a Java object and its data source representation.

Describes Java objects that you map to tables in a relational database that provides special database data types that correspond more closely to object types. Applicable only to the relational databases that TopLink supports that provide these special data types.

A collection of mappings, which describe how the attributes and relationships for that class are stored in the database

The primary key information (or equivalent) of the data source

A list of query keys (or aliases) for field names

Information for sequence numbers

A set of optional properties for tailoring the behavior of the descriptor, including support for caching refresh options, identity maps, optimistic locking, the event manager, and the query manager

There is a descriptor type for each data source type that TopLink supports. In some cases, multiple descriptor types are valid for the same data source type. The type of descriptor you use determines the type of mappings that you can define.

Table 16-2 summarizes the relationship between project, descriptor, and mappings.

16.2.2 Descriptors and Inheritance

Inheritance describes how a derived (child) class inherits the characteristics of its superclass (parent). You can use descriptors to describe the inheritance relationships between classes in relational, EIS, and XML projects.

In the descriptor for a child class, you can override mappings that have been specified in the descriptor for a parent class, or map attributes that have not been mapped at all in the parent class descriptor.

16.2.3 Descriptors and CMP and BMP

You can use descriptors to describe the characteristics of entity beans with container-managed or bean-managed persistence.

When mapping enterprise beans, you create a descriptor for the bean class: you do not create a descriptor for the local interface, remote interface, home class, or primary key class.

When using TopLink Workbench, you must define the project with the correct entity bean type (such as entity beans with container-managed or bean-managed persistence) and import the ejb-jar.xml file for the beans into the TopLink Workbench project.

For CMP projects, you use the ejb-jar.xml file to define the bean's mapped attributes. A descriptor of a bean with container-managed persistence contains a CMP policy used to configure CMP-specific options.

Note:

For EJB 3.0 projects, you can use annotations to define the bean's mapped attributes.

16.2.3.1 Nondeferred Changes

By default, TopLink defers all changes until commit time: this is the most efficient approach that produces the least number of data source interactions.

Alternatively, you can configure an entity bean's descriptor for nondeferred changes. This means that as you change the persistent fields of the entity bean, TopLink CMP modifies the relational schema immediately.

Using nondeferred changes, you can achieve backward compatibility with the native behavior of some EJB containers. You can also accommodate advanced applications that rely on the database and entity changes being synchronized for such things as triggers or stored procedures based on transient state within the transaction, deletion and creation of rows with the same primary key, or other complex queries that depend on transient transaction state.

Nondeferred changes have the disadvantage of being the least efficient approach: they produce the greatest number of data source interactions.

When you configure TopLink CMP to support nondeferred changes, TopLink will continue to handle constraints for mapped relationships among entity beans with the same deferral setting. However, you are responsible for handling any errors that result from making changes to a class that is not deferred, but related to a class that is deferred when a constraint exists between these two classes.

Note:

When you configure a descriptor for nondeferred changes, TopLink CMP does not apply nondeferred changes to dependent objects. Dependent objects are subject to default deferred changes: the relational schema is not modified until commit.

However, if you have non-null foreign key constraints in your database, doing a data source modification after the ejbCreate method executes can cause problems. To get around this, some application servers, such as, for example, OC4J, allow you to create new objects after the ejbPostCreate method executes, and rely on the container to resolve the foreign key constraint.

16.2.3.3 Inheritance

16.2.4 Fetch Groups

By default, when you execute an object-level read query for a particular object class, TopLink returns all the persistent attributes mapped in the object's descriptor. With this single query, all the object's persistent attributes are defined, and calling their get methods returns the value directly from the object.

When you are interested in only some of the attributes of an object, it may be more efficient to return only a subset of the object's attributes using a fetch group with which you can define a subset of an object's attributes and associate the fetch group with either a ReadObjectQuery or ReadAllQuery query.

16.2.5 Descriptors and Aggregation

Two objects–a source (parent or owning) object and a target (child or owned) object–are related by aggregation if there is a strict one-to-one relationship between them, and all the attributes of the target object can be retrieved from the same data source representation as the source object. This means that if the source object exists, then the target object must also exist, and if the source object is destroyed, then the target object is also destroyed.

In this case, the descriptors for the source and target objects must be designated to reflect this relationship.

In EJB 3.0, an aggregate is known as an embeddable. In the EJB 3.0 specification, an embeddable may not contain another embeddable (that is, the EJB 3.0 specification does not support nested aggregates).

16.2.6 Descriptor Customization

You can customize a descriptor at run time by specifying a descriptor customizer–a Java class that implements the oracle.toplink.tools.sessionconfiguration.DescriptorCustomizer interface and provides a default (zero-argument) constructor.

16.2.7 Amendment and After-Load Methods

Using TopLink Workbench, you can associate a static Java method that is called when a descriptor is loaded at run time. This method can amend the run-time descriptor instance through the descriptor Java code API. Use this method to make some advanced configuration options that may not be currently supported by TopLink Workbench.

You can only modify descriptors before the session has been connected; you should not modify descriptors after the session has been connected.

16.2.8 Descriptor Event Manager

In relational and EIS projects, TopLink raises various instances of DescriptorEvent (see Table 119-26 and Table 119-28) during the persistence life cycle. Each descriptor owns an instance of DescriptorEventManager that is responsible for receiving these events and dispatching them to the descriptor event handlers registered with it.

Using a descriptor event handler, you can execute your own application specific logic whenever descriptor events occur, allowing you to take customized action at various points in the persistence life-cycle. For example, using a descriptor event handler, you can do the following:

Synchronize persistent objects with other systems, services, and frameworks.Maintain nonpersistent attributes of which TopLink is not aware.

Notify other objects in the application when the persistent state of an object changes.

16.2.10 Descriptors and Sequencing

An essential part of maintaining object identity is managing the assignment of unique values (that is, a specific sequence) to distinguish one object instance from another. For more information, see Section 15.2.6, "Projects and Sequencing".

When an instance of the Customer class is persisted to XML, the TopLink runtime performs the following:

Gets the default root element.

The Customer class instance corresponds to the root of the XML document. The TopLink runtime uses the default root element specified on the descriptor (customer) to start the XML document. TopLink then uses the mappings on the descriptor to marshal the object's attributes:

<customer>
<name>…</name>
</customer>

When the TopLink runtime encounters an object attribute such as billingAddress, it checks the mapping associated with it to determine with what element (billing-address) to continue:

<customer>
<name>…</name>
<billing-address/>
</customer>

The TopLink runtime checks the mapping's reference descriptor (Address) to determine what attributes to persist:

16.3 Descriptors and Inheritance

Inheritance describes how a derived class inherits the characteristics of its superclass. You can use descriptors to describe the inheritance relationships between classes in relational, EIS, and XML projects.

TopLink recognizes the following three types of classes in an inheritance hierarchy:

The root class stores information about all instantiable classes in its subclass hierarchy. By default, queries performed on the root class return instances of the root class and its instantiable subclasses. However, the root class can be configured so queries on it return only instances of itself, without instances of its subclasses.

Branch classes have a persistent superclass and also have subclasses. By default, queries performed on the branch class return instances of the branch class and any of its subclasses. However, as with the root class, the branch class can be configured so queries on it return only instances of itself without instances of its subclasses.

For example, the FueledVehicle class in Figure 16-1 is a branch class.

Leaf classes have a persistent superclass in the hierarchy but do not have subclasses. Queries performed on the leaf class can only return instances of the leaf class.

In the descriptor for a child class, you can override mappings that have been specified in the descriptor for a parent class, or map attributes that have not been mapped at all in the parent class descriptor.

All leaf classes in the hierarchy must have a class indicator and they must have the same type of class indicator (field or class extraction method).

16.3.1.1 Using Class Indicator Fields

You can use a persistent attribute of a class to indicate which subclass should be instantiated. For example, in a relational descriptor, you can use a class indicator field in the root class table. The indicator field should not have an associated direct mapping unless it is set to read-only.

You can use strings or numbers as values in the class indicator field.

The root class descriptor must specify how the value in the class indicator field translates into the class to be instantiated.

One approach is to configure the root class descriptor with a class indicator dictionary: a collection of key-values that associates a simple key, stored in the class indicator field, with a class to instantiate. Table 16-3 illustrates the class indicator dictionary for the Vehicle class' subclasses, as shown in Figure 16-1.

Table 16-3 Class Indicator Dictionary for the Vehicle Class

Key

Value

F

FueledVehicle

N

NonFueledVehicle

C

Car

B

Bicycle

Another approach is to simply use the class name itself as the value stored in the class indicator field. This avoids having to define unique indicators for each class at the expense of a slightly larger key value (depending on the length of your class names).

16.3.1.2 Using Class Extraction Methods

You can define a Java method to compute the class indicator based on any available information in the object's data source record. Such a method is called a class extraction method.

Using a class extraction method, you do not need to include an explicit class indicator field in your data model and you can handle relationships that are too complex to describe using class indicator fields.

A class extraction method must have the following characteristics:

it must be defined on the root descriptor's class;

it must be static;

it must take a Record as an argument;

it must return the java.lang.Class object to use for the Record passed in.

For example, Table 16-4 lists the rows in the EMPLOYEE table. The Employee class is the base class. Director, Manager, Programmer, and TechWriter classes each derive from the Employee class. However, in your application, instances of Manager, Programmer, and TechWriter classes must be represented as Employee instances and instances of Director must be represented as Director instances. Because there is no a one-to-one correspondence between class and JOB_TYPE field value, the JOB_TYPE field alone cannot serve as a class indicator field (see Section 16.3.1.1, "Using Class Indicator Fields"). To resolve this issue, you could use the class extraction method, shown in Example 16-3.

16.3.4 Aggregate and Composite Descriptors and Inheritance

When configuring inheritance for a relational aggregate descriptor, all the descriptors in the inheritance tree must be aggregates. The descriptors for aggregate and non-aggregate classes cannot exist in the same inheritance tree.

Similarly, when configuring inheritance for an EIS composite descriptor, all the descriptors in the inheritance tree must be composites. The descriptors for composite and noncomposite classes cannot exist in the same inheritance tree.

When configuring inheritance for an XML descriptor, because all XML descriptors are composites, descriptor type does not restrict inheritance.

16.3.5 Inheritance and CMP and BMP

Although inheritance is a standard tool in object-oriented modeling, the EJB specifications prior to 3.0 contain only general information regarding inheritance. You should fully understand this information before implementing EJB inheritance. Be aware of the fact that future EJB specifications may dictate inheritance guidelines not supported by all application servers.

16.4 Descriptors and Locking

This section describes the various types of locking policy that TopLink supports, including the following:

16.4.1 Optimistic Version Locking Policies

With optimistic locking, all users have read access to the data. When a user attempts to make a change, the application checks to ensure the data has not changed since the user read the data.

Optimistic version locking policies enforce optimistic locking by using a version field (also known as a write-lock field) that you provide in the reference class that TopLink updates each time an object change is committed.

TopLink caches the value of this version field as it reads an object from the data source. When the client attempts to write the object, TopLink compares the cached version value with the current version value in the data source in the following way:

If the values are the same, TopLink updates the version field in the object and commits the changes to the data source.

If the values are different, the write operation is disallowed because another client must have updated the object since this client initially read it.

VersionLockingPolicy: requires a numeric version field; TopLink updates the version field by incrementing its value by one.

TimestampLockingPolicy: requires a timestamp version field; TopLink updates the version field by inserting a new timestamp (this policy can be configured to get the time from the data source or locally; by default, the policy gets the time from the data source).

Note:

In general, Oracle recommends numeric version locking because of the following:

accessing the timestamp from the data source can have a negative impact on performance;

time stamp locking is limited to the precision that the database stores for timestamps.

Whenever any update fails because optimistic locking has been violated, TopLink throws an OptimisticLockException. This should be handled by the application when performing any database modification. The application must notify the client of the locking contention, refresh the object, and have the client reapply its changes.

You can choose to store the version value in the object as a mapped attribute, or in the cache. In three-tier applications, you typically store the version value in the object to ensure it is passed to the client when updated (see Section 16.4.6, "Locking in a Three-Tier Application").

If you are using a stored procedure to update or delete an object, your database may not return the row-count required to detect an optimistic lock failure, so your stored procedure is responsible for checking the optimistic lock version and throwing an error if they do not match. Only version locking is directly supported with a StoredProcedureCall. Because timestamp and field locking require two versions of the same field to be passed to the call, an SQL call that uses an ## parameter to access the translation row could be used for other locking policies. For more information, see Section 109.5, "Using a StoredProcedureCall" and Section 109.6, "Using a StoredFunctionCall".

16.4.2 Optimistic Version Locking Policies and Cascading

If your database schema is such that both a parent object and its privately owned child object are stored in the same table, then if you update the child object, the parent object's version field will be updated.

However, if the parent and its privately owned child are stored in separate tables, then changing the child will not, by default, update the parent's version field.

After you enable optimistic version locking cascading, when a privately owned child object is modfied, TopLink will traverse the privately owned foreign reference mappings, updating all the parent objects back to the root.

Optimistic version locking cascading is only applied if the child object is registered in a unit of work.

TopLink supports optimistic version locking cascading for:

object changes in privately owned one-to-one and one-to-many mappings

relationship changes (adding or removing) in the following collection mappings (privately owned or not):

Suppose you register ObjectB in a unit of work, modify an ObjectB field, and commit the unit of work. In this case, ObjectB checks the cache for ObjectA and, if not present, queries the database for ObjectA. ObjectB then notifies ObjectA of its change. ObjectA forces an update on its version optimistic locking field even though it has no changes to its corresponding table.

Suppose you register ObjectA in a unit of work, access its ObjectB to access its ObjectC to access its ObjectD, modify an ObjectD field, and commit the unit of work. In this case, ObjectD notifies ObjectC of its changes. ObjectC forces an update on its version optimistic locking field even though it has no changes to its corresponding table. ObjectC then notifies ObjectB of the ObjectD change. ObjectB then notifies ObjectA of the ObjectD change. ObjectA forces an update on its version optimistic locking field even though it has no changes to its corresponding table.

If you store the locked versions in an object, you must refresh the objects (or their versions) on a failure. Alternatively, you can acquire a new unit of work on the failure and reapply any changes into the new unit of work.

16.4.4 Optimistic Field Locking Policies

Optimistic field locking policies enforce optimistic locking by using one or more of the fields that currently exist in the table to determine if the object has changed since the client read the object.

The unit of work caches the original state of the object when you first read the object or register it with the unit of work. At commit time, the unit of work compares the original values of the lock fields with their current values on the data source during the update. If any of the lock field's values have changed, an optimistic lock exception is thrown.

TopLink provides the following optimistic field locking policies:

AllFieldsLockingPolicy: For update and delete operations, TopLink compares all the fields of the object with all the fields in the data source. If the original value of any fields differ from that in the data source, the write operation is disallowed.

For example, if you changed a customer's last name, TopLink might produce SQL similar to the following:

UPDATE CUSTOMER SET LNAME='new last name' WHERE ID=7 AND LNAME='old last name' AND FNAME='Donald' AND B_DAY='1972' AND CREDIT_RATING='A+' AND EYE_COLOR='Blue'

The main disadvantage of this field locking policy is that it is not the most efficient, especially if the changed object has many attributes.

Note:

This comparison is only on a per table basis. If an update operation is performed on an object that is mapped to multiple tables (multiple table inheritance), then only the changed fields for each table changed appear in the where clause.

ChangedFieldsLockingPolicy: For update operations, TopLink compares only the fields of the object that have changed with the corresponding fields in the data source. If the original value of any such field differs from that in the data source, the write operation is disallowed. TopLink does not make any field comparisons for deletes.

The main advantage of this field locking policy is that it allows concurrent updates of different fields. For example, if one thread updates a customer's last name and another thread updates the same customer's credit rating, and you configure the Customer descriptor with ChangedFieldsLockingPolicy, then TopLink might produce SQL like:

// Unit of work 1
UPDATE CUSTOMER SET LNAME='new name' WHERE ID=7 AND LNAME='old name'
// Unit of work 2
UPDATE CUSTOMER SET CREDIT_RATING='B' WHERE ID=7 AND CREDIT_RATING='A+'

SelectedFieldsLockingPolicy: For update and delete operations, TopLink compares only the selected fields of the object with the corresponding fields in the data source. If the cached value of any such field differs from that in the data source, the write operation is disallowed.

For example, if you select Customer attributes LNAME and CREDIT_RATING, then at run time, TopLink might produce SQL like:

UPDATE CUSTOMER SET LNAME='new name' WHERE ID=7 AND LNAME='old name' AND CREDIT_RATING='A+'

Whenever any update fails because optimistic locking has been violated, TopLink throws an OptimisticLockException. This should be handled by the application when performing any database modification. The application must notify the client of the locking contention, refresh the object, and have the client reapply its changes.

When using field locking policies, a unit of work must be employed for updating the data source.

TopLink provides an optimization for pessimistic locking when this locking is used with entity beans with container-managed persistence: if you set your query to pessimistic locking and run the query in its own new transaction (which will end after the execution of the finder), then TopLink overrides the locking setting and does not append FOR UPDATE to the SQL. However, the use of this optimization may produce an undesirable result if the pessimistic lock query has been customized by the user with a SQL string that includes FOR UPDATE. In this case, if the conditions for the optimization are present, the query will be reset to nonpessimistic locking, but the SQL will remain the same resulting in the locking setting of the query conflicting with the query's SQL string. To avoid this problem, you can take one of the following two approaches:

Ensure that the original version value is sent to the client when it reads the object for the update. The client must then pass the original version value back with the update information, and this version must be set into the object to be updated after it is registered/read in the new unit of work on the server.

Hold the unit of work for the duration of the interaction with the client.

Either through a stateful session bean, or in an HTTP session, store the unit of work used to read the object for the update for the duration of the client interaction.

You must read the object through this unit of work before passing it to the client for the update. This ensures that the version value stored in the unit of work cache or in the unit of work clone will be the original value.

This same unit of work must be used for the update.

The first option is more commonly used, and is required if developing a stateless application.

16.4.6.2 Pessimistic Locking in a Three-Tier Application

If you are using pessimistic locking, you must use the unit of work to start a database transaction before the object is read. You must hold this unit of work and database transaction while the client is editing the object and until the client updates the object. You must use this same unit of work to update the object. If you are building a three-tier Web application (where it is not normally desirable to hold a database transaction open across client interactions), optimistic locking is normally more desirable than pessimistic locking (see Section 16.4.6.1, "Optimistic Locking in a Three-Tier Application").

16.5 Descriptor API

The descriptor API can be used to define, or amend TopLink descriptors through Java code. The descriptor API classes are mainly in the oracle.toplink.descriptors package. These include the following classes:

ClassDescriptor (abstract generic descriptor API)

RelationalDescriptor (relational project-specific API)

DescriptorEventManager (event API)

DescriptorQueryManager (query API)

InheritancePolicy

InterfacePolicy

ReturningPolicy

Locking policies (various optimistic locking policies)

For object-relational data type, EIS, and XML projects, descriptor classes are in the oracle.toplink.objectrelational, oracle.toplink.eis, and oracle.toplink.ox packages, respectively.